The present invention relates generally to thermally conductive polymer compositions. More specifically, the present invention relates to thermally conductive polymer compositions that are highly thermally conductive yet have tensile and flexural properties that are not too brittle for use in common structural plastics applications.
In general, thermally conductive polymer compositions are well known in the prior art. Such compositions are typically formed by loading a variety of thermally conductive fillers including metals, ceramics or carbon into a base polymer matrix, wherein the fillers impart thermal conductivity properties to the overall composition. However, in order to produce a composition that has relatively high thermal conductivity values, a high percentage by volume of filler material must be loaded into the base polymer matrix. While a highly filled composition is typically not problematic in most applications wherein a thermally conductive polymer is utilized, there are applications wherein such highly filled compositions cannot be used. In certain applications, the difficulty arises from the fact that there is generally a very large differential between the tensile modulus of the base polymer resin and the tensile modulus of the filer material that is loaded into the polymer. For example, the difference in tensile modulus (stiffness) between metallic fillers and a typical base polymer resin is on the order of 30 times, while with ceramics the difference can be between 30-100 times and with carbon the difference can be as much as 100-500 times. The introduction of these fillers into the polymer matrix therefore results in a drastic increase in the stiffness of the overall composition. Thus, while higher filler loadings produce a desirable increase in the thermal conductivity of the composition, high filler loadings also tend to greatly increase the resultant tensile modulus of the composition resulting in a finished composition that is quite stiff. This is true regardless of the physical form of the filler (e.g. fiber, particle, sphere, etc.) and regardless of whether the filler is a continuous or discontinuous phase in the finished composite
Further, while the addition of filler to the polymer directly impacts upon the tensile properties of the composition, because the tensile modulus is governed by the volume fraction of the filler additive, the strength of the composition is not impacted in the same manner. The overall strength of the composition is instead influenced by the physical size and shape of the filler that is selected, the dispersion of the filler and the wetting out of the filler by the polymer matrix. If the selected filler does not have a high aspect ratio and a good wet out by the polymer matrix during compounding, the strength of the filler does not necessarily translate into an increase in the strength of the composition. Accordingly, a composition that has a high volume loading of fillers will have the benefit of increased thermal conductivity and stiffness but only a small increase (or often a decrease) in tensile strength, thereby producing a material with a low elongation to break ratio. In other words, a very brittle composition. Below are two examples illustrating the properties of the individual materials in the composition as compared to the resultant material properties of the composition itself: